Microelectronic devices, such as various types of integrated circuits, are generally hermetically sealed in a package to protect the microelectronic device from being damaged and from the ambient. One type of package used comprises a ceramic housing which has a cavity in a surface thereof in which the microelectronic device is mounted with a cover plate extending over the cavity and hermetically sealed to the housing. The cover may be a metal plate or a plate of an insulating material, such as glass or ceramic. The cover plate is generally sealed to the housing by a solder which adheres to the material of both the housing and the cover plate and which will withstand the operating temperature of the microelectronic device. Often, the housing and/or cover plate is provided with an adherent metal layer to which the solder will bond so as to provide the desired hermetic and mechanical bond between the housing and cover plate.
A problem with using a solder for sealing the cover plate to the housing arises from the high temperature required to melt the solder when bonding the cover plate to the housing. There are some microelectronic devices that cannot stand such high temperatures and which can be damaged during the bonding of the cover plate to the housing. For example, certain types of solid-state image sensors include an optical filter layer or a lenslet of a resin which will melt or degrade at the high temperatures required for melting solders. Therefore, it is desirable to have a sealing means for bonding a cover plate to a housing of a microelectronic package which can achieve the bond at low temperature which do not adversely affect the microelectronic device in the package. One such sealing means is described in U.S. Pat. No. 4,895,291 to E. J. Ozimek et al, issued Jan. 23, 1990, entitled "Method of Making a Hermetic Seal in a Solid-State Device". However, a probelm with the sealing means described in this patent is that it will melt at the same low temperature at which it is formed. Thus, if the microelectronic device is subjected to that low temperature the bond will be destroyed. Therefore, the sealing means not only must be capable of being formed at relatively low temperatures, and providing both a good machanical and hermetic seal between the cover plate and housing, but also must be capable of withstanding the operating temperature of the microelectronic device which can be higher than the temperature at which the bond is initially formed.